System and method for contraband detection using nuclear quadrupole resonance

ABSTRACT

A system for detecting a target substance within a class of explosives and narcotics containing quadrupolar nuclei through the use of nuclear quadrupole resonance (NQR). The system applies an RF signal to a coil ( 34 ) to excite the substance under test. If the target material is present, an NQR signal will be picked up by the same coil, That signal is compared with known NQR signals in frequency and amplitude. A signal is displayed in an appropriate way if a threshold value of the NQR signal is equalled or exceeded. The empty coil is statically tuned by means of adjusting the location or capacitance values, or both, of static tuning capacitors ( 102 ) in the coil. The coil is tuned after the specimen is inserted into the coil by means of an auto-tune feature ( 36 ). Effective RFI shielding ( 37 ) is provided to prevent external contaminating signals from being detected by the coil and to prevent RF signals from escaping from the scanner. The invention also includes the method for performing tests with the system.

This is a continuation-in-part of application Ser. No. 08/400,679, filedMar. 8, 1995, now U.S. Pat. No. 5,592,083, issued Jan. 7, 1997.

TECHNICAL FIELD

This invention relates generally to a bulk substance detection systemfor detecting concealed explosives and narcotics, and more particularlyto a practical system and method for such contraband detection employingnuclear quadrupole resonance (NQR).

BACKGROUND ART

Earlier work in detecting contraband substances centered on the subjectof nuclear magnetic resonance (NMR). Work in this area is reflected inU.S. Pat. Nos. 4,166,972, 4,296,378 and 4,514,691. A drawback of NMR isthat is requires relatively large magnets. Magnets are relativelyexpensive, would likely cause personnel to be exposed to large staticmagnetic fields, and could damage magnetically recorded material.

Another attempt at explosives detection employed thermal neutronanalysis (TNA), which can detect nitrogen in any form. Although it coulddetect explosives, it was also triggered by nitrogen-rich nylon andwool, and other innocuous items. These shortcomings resulted in a highrate of false positives. Because it employed potentially hazardousradioactive emissions, TNA systems were also required to be heavilyshielded. As a consequence, TNA systems were very large, very expensive,and also produced a high rate of false positives.

X-ray screening, commonly used in airports, does not have the sameoverall limitations as TNA. However, it cannot alert the operator to thepresence of explosives or drugs, much less identify them. X-rayscreening can only “see images that the operator must interpretquickly.” Further, X-ray screening emits potentially hazardous ionizingradiation.

With respect to explosives, plastic explosives such as C4 and Semtex,containing RDX and PETN, have an almost infinite variety of possibleshapes and uses for terrorist bombing tactics. Plastic explosives arehighly stable, have clay-like malleability and are deadly in relativelysmall quantities. A small piece of plastic explosive, a detonator, and atrip wire inside a large mailing envelope can cause a deadly explosion.Unfortunately, without close—and potentially dangerous—visualinspection, plastic explosives can be made virtually untraceable.Because of the drawbacks of TNA, NMR and X-ray, as mentioned above, theyhave generally proven ineffective for practical bulk detection of thesetypes of explosives. In particular, detection of sheet explosives,typically having a thickness as small as 6.35 mm (0.25 inch), has notbeen effectively accomplished by prior technologies.

The wide-scale attempts to fight the illegal drug trade indicates thatnarcotics detection is also extremely important. The need for a simpleprocedure for detecting drugs inside sealed containers, mail parcels,and other small packages, quickly and accurately, is immeasurable.Conventional detection methods are time-consuming, costly, and have onlymarginal reliability at best.

NQR is a branch of radio frequency spectroscopy that exploits theinherent electrical properties of atomic nuclei. Nuclei withnon-spherical electric charge distributions possess electric quadrupolemoments. Quadrupole resonance arises from the interaction of the nuclearquadrupole moment of the nucleus with the local applied electrical fieldgradients produced by the surrounding atomic environment.

Any chemical element's nucleus which has a spin quantum number greaterthan one half can exhibit quadrupolar resonance. Many substances(approximately 10,000) have been identified that exhibit quadrupolarresonance, among such nuclei being: ⁷Li, ⁹Be, ¹⁴N, ¹⁷O, ²³Na, ²⁷Al,³⁵Cl, ³⁷Cl, ³⁹K, ⁵⁵Mn, ⁷⁵As, ⁷⁹Br, ⁸¹Br, ¹²⁷I, ¹⁹⁷Au, and ²⁰⁹Bi. It sohappens that some of these quadrupolar nuclei are present in explosiveand narcotic materials, among them being nitrogen (¹⁴N), chlorine (³⁵Cl,³⁷Cl), oxygen (¹⁷O), sodium (²³Na), and potassium (³⁹K). The moststudied quadruple nucleus for explosives and narcotics detection isnitrogen.

In solid materials, electrons and atomic nuclei produce electric fieldgradients. These gradients modify the energy levels of any quadrupolarnuclei, and hence their characteristic transition frequencies.Measurements of these frequencies or relaxation time constants, or both,can indicate not only which nuclei are present but also their chemicalenvironment.

When an atomic quadrupolar nucleus is within an electric field gradient,variations in the local field associated with the field gradient affectdifferent parts of the nucleus in different ways. The combined forces ofthese fields cause the quadrupole to experience a torque, which causesit to precess about the electric field gradient. Precessional motiongenerates an oscillating nuclear magnetic moment. An externally appliedradio frequency (RF) magnetic field in phase with the quadrupole'sprecessional frequency can tip the orientation of the nucleusmomentarily. The energy levels are briefly not in equilibrium, andimmediately begin to return to equilibrium. As the nuclei return, theyproduce an RF signal, known as the free induction decay (FID). A pick-upcoil detects the signal, which is subsequently amplified by a sensitivereceiver to measure its characteristics.

One distinguishing feature of an NQR response is its precessionalfrequency. Two independent factors determine the precessional frequency:the quadrupolar nucleus, and its local crystalline environment. Theremay be one or more characteristic NQR frequencies for each substancecontaining quadrupolar nuclei.

The second distinguishing features are the NQR relaxation times.Relaxation times are a measure of the nuclei's rate of return to theequilibrium state following disturbance by an RF pulse. Relaxation timesare compound-, temperature-, and pressure-specific. Relaxation timesalso determine the repetition rate and timing of RF pulses required forexciting and detecting a specific NQR signal. Relaxation times can be asshort as a few hundred microseconds or as long as several seconds.

Detection of NQR signals normally requires RF transmitting and receivingapparatus. To minimize noise and radio frequency power requirements andimprove receiver sensitivities, conventional NQR systems use a narrowband (high Q) sample coil in both the transmitting and receivingequipment. Even so, several factors can significantly degrade theeffectiveness of detecting NQR signals. Among these factors are: (1) thepresence of conductive materials inside the sample coil; (2) thepresence of materials with a high dielectric constant inside the samplecoil; (3) temperature, which can affect the value of the capacitanceused for tuning and matching the RF coil; and (4) mechanical movement ofthe coil which respect to its surroundings. All of these factors cancause serious de-tuning of the detection apparatus, which in turn,lowers the detection sensitivity of the coil. Accordingly, NQR systemshave largely been limited to small sample laboratory systems with littleor no “real-world” potential.

The NQR energy level transitions are observed primarily in the radiofrequency range. Detection of these transitions requires an RF source toexcite the transition, and an RF receiving mechanism to detect thesignals returning from the nuclei. Normally, the signals appear at apre-defined frequency. An RF coil tuned to, or close to, that predefinedfrequency can excite and/or detect those signals. The signals are ofvery low intensity and can only be observed for a short time,approximately 10 μs to 10 ms. As a consequence, there is a need for anNQR receiver that can be tuned to a (usually) high Q, has very lownoise, and is capable of fast recovery after a high-voltage RF pulse.

Previous work in this area is reflected in U.S. Pat. Nos. 4,887,034,5,206,592, 5,233,300 and 5,365,171. Use of NQR for explosives andnarcotics detection is also discussed in Buess et al., ExplosivesDetection By ¹⁴ N Pure NQR, Advances in Analysis and Detection ofExplosives (J. Yinon (ed.)) pp. 361-368 (1993), and Shaw, NarcoticsDetection Using Nuclear Quadrupole Resonance (NQR), Contraband and CargoInspection Technology International Symposium, Washington, D.C., pp333-341 (1992).

Detection by means of NQR is possible for both explosives and narcotics,partially because they have as a constituent element ⁴N in crystallineform. Particularly with respect to narcotics, this is true of cocainebase, cocaine hydrochloride and heroin based narcotics. Thehydrochloride forms of narcotics, such as cocaine hydrochloride, alsocontain quadrupolar nuclei ³⁵Cl and ³⁷Cl. For example, U.S. Pat. No.5,206,592 discloses the concept at using a planar meanderline coil pastwhich a specimen is passed. The coil is tuned to about the target signalfrequency and applies a pulsed RF signal to the specimen and picks upthe nuclei relaxation signals from target substances present in thespecimen. A CPU is employed to process the received signals and toactivate an alarm when the received signal exceeds a predeterminedthreshold.

Where coils are involved, the Q of the coil is often of majorimportance. Ochi et al., Analysis of a Magnetic Resonance ImagingAntenna Inside an RF Shield, Electronics and Communications in Japan,Part 1, Vol. 77, No. 1, pp 37-45 (1994), teach how to quantify thechange in Q of an MRI antenna with changes in length and diameter of theshield. However, this deals with MRI and not NQR, and is relevant onlyto humans and not to packages or baggage. An automatic tuning system isdisclosed in Butler et al., High-Power Radio frequency Irradiationsystem with Automatic Tuning, Rev. Sci. Instrum., Vol. 53, No. 7, pp984-988 (1982). This tuning system is useful in an NQR spectrometer andin other nuclear resonance experiments involving frequency sweeps. Inthe Butler system, data is predetermined for various frequencies and isnot designed to compensate for an unknown coil loading. U.S. Pat. No.5,209,537 discloses a method for matching antennas in an NMR imagingapparatus for use in producing tomograms.

A significant factor in contraband detection by means of NQR is thatquadrupolar nuclei that are commonly present, and potentially readilyobservable, in narcotics and explosives include nitrogen (¹⁴N) andchlorine (³⁵Cl and ³⁷Cl), among other possible nuclei. Thus, incommercial applications it is necessary to be able to detect quadrupolarnuclei contained within articles of mail, mail bags or airline baggage,including carry-on and checked luggage. While the resonant frequenciesof the nitrogen in these substances differs for each chemical structure,these resonant frequencies are well defined and consistent. By applyingan RF signal to a container having any of these suspected substancesinside, and then detecting any quadrupolar resonance thus engendered bythe application of RF pulses, the identity of the contraband substancecan be easily determined.

DISCLOSURE OF INVENTION

Broadly speaking, this invention provides a practical nuclear quadrupoleresonance (NQR) detector system for improved bulk contraband detection.More specifically, the invention employs the principle of NQR to simplyand relatively inexpensively, with a very low rate of false alarms,detect the presence of explosives and/or narcotic materials withinclosed or sealed packages or within baggage having many other articlesof other materials contained therein. It is particularly effective indetecting contraband materials in sheet form, which are as thin as onequarter inch or possibly even thinner.

The invention is a system for detecting a target substance within aclass of explosives and narcotics containing quadrupolar nuclei in aspecimen employing the phenomenon of nuclear quadrupole resonance (NQR),said system comprising: a sequence controller having means for providingtiming and programming pulses to said system; a radio frequency (RF)subsystem comprising a variable frequency RF source to provide pulsed RFexcitation at a frequency generally corresponding to predeterminedcharacteristic nuclear quadrupolar resonant frequency of the specimen,wherein nuclear quadrupolar resonant frequency is the frequency ofnuclear precession due to quadrupolar interaction with molecularelectric field gradients; a detection head subsystem comprising: asingle turn distributed RF coil sheet shaped and configured to define acavity of predetermined volume therein and to receive the specimenwithin the cavity defined by said RF coil, said cavity having a firstend and a second end, the RF signal from said RF source beingtransmitted within said cavity and being uniformly applied to thespecimen within said RF coil cavity and generating a uniform fieldwithin said cavity, said RF coil also functioning as the pickup coil forthe NQR signals from the specimen and providing an NQR output signal;apparatus for tuning said RF coil to about the desired characteristicnuclear quadrupolar resonant frequency for the specimen under test; andan electrically conductive RF shield surrounding and spaced from andelectrically isolated from said RF coil, said RF shield being shaped andconfigured to provide electromagnetic interference and radio frequencyinterference (EMI/RFI) shielding from external noise and to prevent RFand magnetic flux from escaping from said RF coil cavity and RF shieldcombination, said RF shield configuration being longer than said RFcoil, thereby extending beyond both said first end and said second endof said coil, said RF shield being an electrically integral part of saidRF coil to improve the Q and the efficiency of said RF coil andcontributing to the uniformity of flux field applied to the specimenwithin said RF coil, said RF coil and RF shield together forming ascanner; a signal capture and data processing subsystem having a digitalsignal processor and comprising: means for receiving the NQR outputsignal from said RF coil; memory means storing characteristics of NQRsignals from at least one target substance in the class explosives andnarcotic compounds; means for processing the NQR output signal from saidRF coil; and means for comparing characteristics of the processed NQRoutput signal with the characteristics in memory and emitting a finaloutput signal; and a display device receiving the final output signalfrom said signal capture and data processing subsystem and, in responsethereto, said display device selectively indicating the presence of thetarget substance, the absence of the target substance, and anintermediate result when conditions of the received signal from thespecimen indicate that further testing is necessary.

The invention is a also a method for detecting a target substance withina class of explosives and narcotics containing quadrupolar nuclei in aspecimen, said method employing the phenomenon of nuclear quadrupoleresonance (NQR) in a detection system and comprising the steps of:forming a scanner comprised of a single turn distributed RF coil sheetshaped and configured to define a cavity of predetermined volume thereinand to receive the specimen within the cavity defined by the RF coil,the RF coil being surrounded by an electrically conductive RF shieldwhich is spaced from and electrically isolated from the RF coil, theshield being shaped and configured to provide electromagneticinterference and radio frequency interference (EMI/RFI shielding fromexternal noise and to prevent RF and magnetic flux from escaping fromthe scanner, the RF shield configuration being longer than the RF coil,thereby extending beyond the ends of the RF coil, the RF coil and RFshield being an integral combination portion of the scanner and beingdesigned to improve the Q and the efficiency of the RF coil, the RF coiland RF shield contributing to the uniformity of flux field applied tothe specimen when it is inserted within the RF coil cavity; enteringknown characteristics of NQR signals of target substances in memory in adata signal processor in the detection system; providing preciselyprogrammed timing pulses to the detection system; inserting the specimenwithin the cavity formed in the RF coil; then automatically tuning theRF coil to maximum power transfer efficiency for RF signals transmittedwithin the RF coil cavity; providing excitation RF pulses of apredetermined frequency to the RF coil; transmitting the RsB pulses intothe cavity formed by the RF coil and creating a uniform flux fieldwithin the RF coil to which the specimen is subjected; detecting by theRF coil the NQR signals emitted by target substances within thespecimen; processing the NQR signals and comparing them to signalcharacteristics in memory to determine whether the detected NQR signalsindicate the presence of a target substance; and selectively indicatingwhether the target substance is present in the specimen, whether thetarget substance is absent from the specimen, and whether conditions ofthe received signal indicate that further examination is necessary.

The invention provides a commercially practical system employing theknown properties of the substances and the known principles of NQR todetect and identify contraband products which may be hidden insideairline baggage or concealed in a variety of packaging. RF pulses areapplied to an RF coil in which the specimen resides for purposes of thetest. An appropriately formed RF shield prevents stray signals fromentering or leaving the cavity in the coil so that the test results canbe reliable and external RF radiation is insignificant. Becauseinsertion of the sample in the RF coil causes de-tuning, an automatictuning system is provided to re-tune the RF coil to provide optimumperformance under a range of coil loading conditions. This tuning systemalso corrects for the possible degradation of NQR response signalscaused by temperature changes and others of the factors set out above inthe Background.

An analog signal is converted to digital form and is sent to a digitalsignal processor. The digitized signal is digitally filtered andcompared with a predetermined threshold level. Alternatively, once thesignal is apodized and Fourier transformed, it occurs as a quadrature“spike” at or close to 0 Hz in the frequency spectrum, and is thenfiltered and compared to the known signal of the material to bedetected. The signal from a digital signal processor is applied to adisplay device which indicates whether the package is clean, hascontraband, or needs further inspection.

In a practical system, the presence of other conductive material in thepackage being inspected may cause acoustic ringing as a result of theapplied RF pulses. The system of the invention has provisions to filterout such acoustic ringing when it occurs so that the NQR signal isisolated and is not hidden in the ringing signal.

BRIEF DESCRIPTION OF DRAWING

The objects, advantages and features of this invention will be morereadily appreciated from the following detailed description, when readin conjunction with the accompanying drawing, in which:

FIG. 1 is a block diagram of the basic system of the invention;

FIG. 2 is perspective view of an actual device in accordance with theinvention for small package inspection incorporating the FIG. 1 systemtherein;

FIG. 3 is a partially cut away perspective view of the scanner portionof the FIG. 2 device;

FIG. 4 is a partially cut away perspective view of a baggage inspectiondevice constructed according to the invention;

FIG. 5 shows the auto-tune subsystem in greater schematic detail;

FIG. 6 is a flow diagram of the operation of the auto-tune subsystem ofFIG. 5; and

FIG. 7 is a perspective view of the RF coil, core and cavity of theinvention, showing the static tuning capacitors in the coil gap.

BEST MODE FOR CARRYING OUT THE INVENTION

NQR is a linear spectroscopy, that is, the signal strength is directlyproportional to the quantity of contraband material containingquadrupolar nuclei. Because the NQR frequencies of different compoundsare quite distinct, the system of this invention does not encounterfalse alarms from the NQR signals of other benign materials. Forexample, ¹⁴N NQR absorption frequencies from crystalline materials arevirtually unique. When looking for the nitrogen signal at the NQRfrequency of RDX, for example, only nitrogen in RDX will be detected. Ifother compounds containing ¹⁴N are in the same parcel as the RDX, thoseother compounds would not be identified. The frequency resulting fromNQR in a target substance will be sharply defined, while other¹⁴N-containing substances would not provide a sharp peak NQR response.Another factor of importance is that NQR is a bulk detector, that is,sheet, bulk or distributed materials are equally detected.

The unique NQR resonance frequencies of a large number of compounds havebeen identified and recorded. The frequency information is stored in amemory in the system of the invention and provides a database forcomparison of detected signals. For general reference purposes, the NQRfrequencies of quadrupolar nuclei are generally within the range of 0.5MHz to 5 MHz.

It is important to understand that explosives and narcotics have“fingerprints” that are different from innocuous substances. To thwartanalysis by NQR, it would require the impossible task of altering thechemical structure of the contraband, and the laws of nature cannot bealtered. Thus, to change a substance so that the elements of interest inthe particular contraband could not be detected by means of NQR wouldrequire changing the chemical composition and make it other than thecontraband itself.

With reference now to the drawing, and more particularly to FIG. 1 whichshows the system, block 21 is the sequence controller subsystem. Thissubsystem provides precise timing and other control functions for allother elements and subsystems of the invention. It generally wouldcomprise a microprocessor-based device which provides means to downloadand initialize the sequence control information to all other subsystems,and would include appropriate data storage or memory means. It alsostores information on the results of individual scans for futurereference. As one specific embodiment, the microprocessor based controland storage device may be a personal computer (PC) with a hard disk.

The sequence controller subsystem also includes a pulse programmer whichis a high-precision, high-resolution device that runs off the standardcomputer bus. The pulse programmer provides the precise sequence controlrequired for correct operation of all other major components in the NQRscanner of the invention. In combination with the personal computer, italso provides the precisely defined pulses and triggers to activate thesubsystems to which it is connected and which will be discussed indetail below.

Radio frequency (RF) subsystem 22 has several functional elementsincluding RF signal source 23, RF power amplifier 24, receiver RFpreamplifiers 25, receiver RF amplifier 26 and detectors 27 and 28. Thedetectors are here shown as phase-sensitive detectors. A 90° degreephase shift generator 31 is also part of the RE subsystem. This is oneembodiment of the invention and is used when detectors 27 and 28 arephase shift detectors. Other types of detectors could be employed andthe phase shift generator would not be required. Conventional amplifierprotection devices 29 are also part of the RF subsystem. They aretypical RF amplifier-related elements and need not be described indetail here.

RF signal source 23 provides either continuous or pulsed RF excitationat a frequency corresponding to the resonant frequency of the samplematerial. For example, RDX-based plastic explosives have a resonantfrequency of approximately 3.410 MHz while PETN-based plastic explosiveshave a resonant frequency of approximately 890 KHz. The excitationsource is fed into amplifier 24 of sufficient power rating to generateabout 1 gauss of RF magnetic field within the coil. The excitationfrequency need not be exactly the same as the target substance NQRfrequency but it should be within about 500-1000 Hz. The RF excitationfor NQR detection could be a single pulse of 10 μs-500 μs duration,depending on the substance being tested for. Such a single pulse couldcause an NQR return, but the nuclei may not have reached a steady stateof precess so the NQR return might not be sufficiently strong to bedetectable or useful. For a letter bomb scanner, approximately threeseconds of RF pulses at a repetition rate of 667 pulses per second,meaning a train of 2000 pulses having a pulse width of 200 μs each,would preferably be applied. The pulse repetition rate can range between300 Hz and 2 KHz. This would result in a series of NQR signals which areadded and averaged in digital signal processor 44. This is anapplication of the conventional technique where target signals are addedlinearly while noise adds randomly, thereby building a clearly definablepulse by improving the signal-to-noise ratio (SNR). Any method toimprove SNR might advantageously be used.

The power requirements of the invention are generally proportional tothe detection coil volume. An explosives scanner for mail packages witha 25 liter detector coil volume might have an RF power amplifier ratedat about 25 Watts, peak value, for example. The amplifier produces auniform RF field of about 1 gauss over the entire 25-liter volume. Inother applications, such as in narcotics detection, the RF field may begreater than this value. For airline baggage, an explosives detectionhead of about 300 liters (10 ft³) volume within the coil requires a 1 to2 KW RF power amplifier. These parameters are provided for referencepurposes and are not meant to define or limit the actual characteristicsof a practical NQR system.

The RF excitation pulses are fed from amplifier 24 into detection head33, the operation of which will be discussed below. After the sample inthe detection head has been excited by the RF pulse, a short RF coil“ring-down” or dead time occurs, during which the receiver is “deaf,”before sensing occurs. This ring-down time could, for example, be 500μs. Then RF coil 34 detects the NQR signals and the response isamplified by low-noise, high-gain preamplifiers 25 having a gain of 20to 30 dB, and a noise figure of 1 to 2 dB. Examples of suchpreamplifiers are Anzac Model AM-110 and Mini-Circuits Model ZFL-500LNS.

In the package or letter scanner size configuration of the invention,after the received signal has been sufficiently amplified by RFamplifiers 25 which, together with amplifier protection components 29,include appropriate conventional filter functions, the received signalis fed into two phase sensitive detectors 27 and 28, having referencesignals shifted 90° from each other by means of phase shift element 31.Note that reference RF signal from RF source 23 is applied to phasesensitive detector 27 while the reference signal to phase sensitivedetector 28 passes through phase shift element 31. The two mutuallyphase-shifted analog signals are then fed into signal-capture and dataprocessing subsystem 41, which will be discussed below.

Detection head subsystem 33 is comprised of four main components. Theseare RF coil 34, an RF probe circuit which is RF tuning and matchingnetwork 35, auto-tune subsystem 36 and RF shield 37. The detection headserves two primary purposes. One is to produce a homogeneous RF field inthe RF coil. The other is to receive the raw NQR signal, if present,from the item under investigation. The manner in which a homogeneousfield is ensured within the RF coil cavity to achieve uniform tip anglesin the nuclei of the target substances will be described with respect toFIG. 7.

RF coil 34, which may also be referred to as an antenna, is made of ahighly conductive material, such as copper. The conductor should have athickness in the order of at least five times the skin depth of thematerial of the conductor at the operational frequency. This ensures aminimal amount of resistance to the flow of current when the coil isenergized with RF. A 25 liter detection volume (for a mail scanningdevice) has a single turn, high-Q, 0.25 mm (0.01 inch)-thick copper coilmade of sa single sheet. The skin depth of copper at 3.4 MHz is about0.025 mm (0.001 inch) and the skin depth of copper at 900 KHz is about0.051 mm (0.002 inch). Direct coil tuning results in an increasedoverall efficiency for the mail scanning embodiment of the invention.The single-turn, high-Q coil, when no sample is present, that is, thecoil is empty, requires approximately 30,000 pF of capacitance fortuning at about 3.4 MHz in order to detect the ¹⁴N resonant frequency ofRDX explosives. Using a series of switches to add or remove capacitancein order to re-tune the coil under differing load conditions, it hasbeen determined that it would be useful for the system to be re-tunablefor a 10% change in tuning capacitance. In this particular application,the coarse tuning increments in capacitance were selected to beapproximately 80 pF, and in the fine tuning mode, 10 pF. The RF signalsource and amplifier (23 or 24) of RF sub-system 22 used to exercise theauto-tune subsystem are the same as those used to excite the RF coil forsubstance detection purposes. Details of the auto-tune subsystem are setout hereinbelow.

The basic tuning of the coil to create a uniform field within the RFcoil cavity during the transmit mode is necessary for optimum operationand sensitivity of the system. This homogeneity is important because itis highly desirable to cause uniform tip angles of the nuclei throughoutthe expected measurement volume. Of course, the measurement volume ofinterest is the target substance (contraband) within the specimen orsample in the coil cavity. Uniform sensitivity in the receive mode, dueto reciprocity, to the nuclear precession-generated fields, is equallyimportant. A “hole” in the field can exaggerate the difficulty indetecting a target substance by reducing the effect of the generatedsignal (less than complete and uniform nuclei tipping) and at the sametime resulting in a reduced received signal.

Further details of these concepts follow.

It is desirable for a volume detection system to have a uniformsensitivity throughout the detection volume. With NQR this can beassured by having a uniform RF flux field in that detection volume. Aregion of reduced field may cause a reduction in sensitivity due toreduced tipping of the nuclei during the time of the excitation ortransmitted pulse. By reciprocity, the antenna will be less sensitive tothe nuclear induction signals in those regions of reduced field duringthe receive mode. The effects of changes in tip angle and the changes inreceiver sensitivity are cumulative. Thus, a 25% reduction in the effectof the excitation pulse results in about a 50% reduction in receivereffect, or overall sensitivity.

With reference now to FIG. 7, for a single turn coil 34 with a length Lmuch greater than the height X or the width Y, a uniform RF field willexist in central region 52 provided that the static tuning capacitanceis distributed or spaced generally uniformly along gap 101 of the coil.Improper placement of the distributed static tuning capacitance in thecentral region can cause variations in the field in that region. For acoil with a length not substantially greater than the lesser of theheight or width; the field near the ends will be less if the capacitanceis uniformly distributed along the entire gap. The field at the ends canbe made more uniform by increasing the relative portion of the totalcapacitance placed near the ends of the gap. A multiplicity of statictuning capacitors 102 are shown in gap 101. They are shown evenly spacedhere for purposes of simplicity, and because their placement andrespective values are determined when the detection system isconstructed. For reference purposes, it was previously stated that thetotal capacitance for an empty RF coil is about 30,000 pf for statictuning at about 3.41 MHz, the ¹⁴N resonant frequency of RDX explosives.Of course, different target substances have different crystallinestructures and different ¹⁴N resonant frequencies, so the distributedstatic tuning capacitance would be different. The sizes and spacings ofcapacitors 102 are determined by the procedure which follows.

By way of example, the ideal tip angle for RDX is 117° and its nuclearresonant frequency is 3.410 MH. The total capacitance necessary in thecoil is determined according to the equation: $\begin{matrix}{C_{Total} = {{C_{0}\left( \frac{f_{actual}}{f_{0}} \right)}^{2} = {C_{0}\left( \frac{f_{actual}}{3.410} \right)}^{2}}} & {{Eq}.\quad 1}\end{matrix}$

The procedure for determining C_(total), the desired capacitance, is toconnect a probe connected to an inductance meter across coil gap 101 andmeasure the inductance. By approximation, a multiplicity of capacitors,totalling approximately 30,000 pf, are distributively connected acrossthe gap as shown in FIG. 7. Then with an impedance meter, the resonantfrequency of the coil is measured to provide f_(actual). That frequencyis plugged into Eq. 1, the division made, the result squared, and thatnumber is multiplied by 30,000 (C₀), giving a closer approximation forC_(total). This static tuning is repeated until the resonant frequencyof the coil is about 3.410 MH. Then the capacitor distribution isadjusted to achieve homogeneity of the field within the coil cavity, asset out below.

To determine if the capacitor placement is correct for the empty cavitycoil, the RF coil is connected to a signal generator tuned to about theresonant frequency of the coil for the specified target substance and asmall pickup loop connected to a conventional impedance meter is used tomap the RF flux field inside the coil (the axis of the loop is parallelto the axis of the coil). If the field is not sufficiently uniform alongthe axis, some capacitance is removed from areas of high field andtransferred to areas of low field. The process of measuring the fieldand transferring capacitance is continued until the desired fieldhomogeneity is achieved.

Auto-tune Subsystem

Apparatus for automatic fine tuning of the NQR detection coil/head underadverse conditions is shown in FIG. 5. Within sequence controller 21 issoftware or control programming 91 for auto-tune subsystem 36. Theauto-tune subsystem is preferably incorporated within RF shield 37, asare RF coil 34 and matching network 35. Input/output line 92 connectsthe tuned RF coil to the amplified RF excitation signal and connects thecoil as the receiver of the NQR signals to ¼ wave line 38 (FIG. 1).

The system consists of a series of fixed value capacitors 93 switched byan equal number of vacuum relays 94. The amount of capacitance switchedinto the tuning circuit is determined by measuring the amount of powerbeing transferred from RF amplifier 24 to RF detector coil 34 (or, moreprecisely, the amount of “forward” to “reflected” power.) The means tomeasure this power transfer efficiency consist of a variety of common RFtechniques. For one application, a directional watt meter is used tomeasure the amount of “forward” to “reflected” power. Based on the powertransfer efficiency, capacitors are switched in or out of the circuit tomaximize power transfer efficiency from the RF amplifier to the RF coil.The system is thus re-tuned to provide the most efficient and mostsensitive RF coil. Once the state of tune of the RF coil has beendetermined by the values of the forward and reflected power, the coil isre-tuned by switching in capacitance according to the algorithmdescribed below.

Tuning of the RF coil consists of two stages: coarse tuning and finetuning. A flow diagram for the sequence is shown in FIG. 6. The value of“C” in FIG. 5 has been chosen to be 10 pf, so each capacitor is amultiple of “C.” Other values could be assigned as desired.

Coarse Tuning

Both the forward and reflected power are measured. If reflected power isgreater than a predefined percentage of forward power, then the systemself-adjusts to coarse tuning by making the large jumps mentioned above(by increasing capacitance until reflected power drops below the maximumvalue of reflected power that the fine tuning mode can handle). Whenthat condition is reached, then the system goes into fine tuning. Theupper limit of the size of the capacitance jump is determined by thecapacitance range of the fine tuning subsystem. When the reflected powerdrops below a preset upper size limit, then the system will begin finetuning. This is the “start fine tuning mode” point.

Fine Tuning

After taking a step (either increasing or decreasing capacitance) thereflected power is measured again. If reflected power has increased andthe direction (i.e., increase or decrease of capacitance) has beenreversed from the previous step, then the system goes back one step tothe “start fine tuning mode” point. Fine tuning begins again, only thistime in the opposite direction (that is, adding capacitance instead ofsubtracting it). If, however, the reflected power did not increase, thenanother step is taken in the same direction (adding or subtractingcapacitance). This process continues until another reversal of thedirection is encountered. At this stage the system goes back one stepand the fine tuning is complete. The reflected power is now at aminimum. The forward power is measured and compared to a pre-definedvalue, to ensure correct functioning of the RF transmitter.

Auto-tune subsystem 36 performs two major functions. One is to re-tunethe RF coil to provide optimum performance under a range of coil loadingconditions. Secondly, it determines the state of the tune by comparingit to a pre-defined “zero” setting. The system consists of a radiofrequency power source, a directional watt-meter and switched capacitorsto vary tuning reactance. Control unit 21 operates the RF power source,measures the reflected power and then varies the tuning reactance untila minimum in reflected power is reached. The system's ability to tunethe sample coil directly results in increased overall efficiency.Antenna tuning systems commonly used in radio electronics areunnecessarily complex for coil fine tuning in NQR applications. Theyalso have certain inefficiencies for NQR applications: they cannot tunethe coil directly, and they experience higher feed line losses, whichcan contribute to noise. Furthermore, antenna tuning systems tend to betoo general in terms of what is being matched (for example, tuningrange).

RF probe 35 is a matching network and Balun which provides tuning andmatching of the coil, and also protects preamplifiers 25 from the highvoltages in the coil during RF excitation. RF probe 35 matches RF coil34 to a 50 Ω unbalanced input. This makes the coil look like a 50 Ωtransmitter/receiver and is conventional matching technology. Thefunction of ¼ wave line 38 is to isolate the receiver from thetransmitter. Transmitter isolation diodes 39 and 40 have a relatedfunction. The auto-tune subsystem determines the state of the tune of RFcoil 34 in the detector head by matching the RF coil to its load in thedetection volume. It measures the amount of power transferred directlyto the RF coil (the “forward” power), and the amount of power reflectedback due to losses in the circuit and mis-tuning (the “reflected”power). Once the tuning state is determined by comparing the values ofthe forward and reflected powers, the coil is re-tuned by switchingcapacitance according to a predetermined sequencing as has beendiscussed above.

When coil 34 is loaded with a package of unknown contents, it becomesde-tuned. In one application of this invention, to re-tune the coil,eight vacuum relays switch the capacitors arranged in pF values ofpowers of two, that is, 10, 20, 40, 80. This particular arrangement iscapable of producing 256 values of capacitance for re-tuning the system,with a maximum total of 3000 pF. Rather than overloading the system withone relay for each value of capacitance, this power arrangementminimizes the number of relays needed to produce a given value ofcapacitance (eg. 10+20=30; 20+80=100, etc.), and affords very fastoperational speed. It should be noted that the same algorithm can beused with a continuously-variable capacitance system. A stepper motorcould be employed and the actual tuning sequence would be very similarto that described for discrete, direct capacitor tuning. The direct coiltuning capacitance arrangement described above is preferred for thisinvention.

Using capacitors switched by vacuum relays requires a “settling time” ofabout 6 ms or less to allow the relays to operate and for the reflectedpower to achieve a steady-state value. The benefit in overall systemruggedness, efficiency, reliability, and small size due to the fixedswitch capacitor scheme overcome any possible advantage in precisiontuning which might have been achieved using the more conventionalvariable capacitors. However, because the system uses switching commandscontrolled by a computer operated sequence controlling device, it canget exact information on the amount of system de-tuning.

This tuning sub-system offers improved sensitivity for NQR systems byoptimum automatic fine tuning of the sample coil (RF coil). Previousdevelopments in coil fine tuning required manual tuning of the system,which is acceptable for the laboratory but undesirable for field use.This system offers the advantage of automatic tuning based on fixedcapacitors switched by vacuum relays (designed for high RF switching)rather than bulkier and slower variable capacitors. The proposed systemmeasures changes in coil loading, a feature not available on otherdetection systems. The system is faster and easier to use than amanually tuned sample coil, and provides information about thestate-of-tune of the RF coil which can give an indication of thecontents of the coil (the sample). The system also gives the controlunit an indication of the performance of the RF amplifier.

Physical configurations of the scanner of the system will be describedwith respect to FIGS. 2, 3 and 4. RF coil 34 is a hollow rectangulartube of thin sheet conductive material, as previously described, formedon thin-walled rectangular insulator 51 (See FIG. 3). Shield 37 is aconductor in the shape of a rectangular copper (or other highlyconductive material) sleeve enclosing the coil and spaced from it by adistance of about one half the length of the shortest side of the coil.The shortest side of the coil is represented by distance “X” in FIG. 3and the spacing is preferably X/2. As an example of actual size, X is12.7-15.2 cm (5 to 6 inches), so the spacing between coil 34 and shield37 would be about 6.35-7.62 cm (2.5 to 3.0 inches). Another significantmeasurement is the distance between the edge of coil 34 and opening 52through which the item to be tested is inserted. That is the same X/2distance, or about 6.35-7.62 cm (2.5 to 3.0 inches). The RF shieldprovides the coil and probe units, that is, the structure within the RFshield, with the necessary EMI/RFI (electromagnetic interference/radiofrequency interference) shielding from external noise. At the same time,the structure inhibits RFI from escaping from the specimen testingcavity. This configuration has been optimized to provide the bestbalance between noise isolation of the coil, loading of the coil, andminimization of the total system volume.

To complete the dimensions of the scanner of FIG. 3 for purposes ofexample, the long dimension of the rectangular cavity at the opening maybe 16 inches, and the length of the cavity within the coil may be 61 cm(24 inches). Surrounding shield 37 may have a depth of 25.4-29.2 cm (10to 11.5 inches), a width of about 51-56 cm (20 to 22 inches), and afront-to-back length of at least about 68.6 cm (27 inches). The volumeof the cavity would be about 26 liters (2000 in³). The scanner describedabove may be referred to as a box with a cavity therein, having externalaccess opening 52 to the cavity.

Different arrangements are necessary for the front and back of the coil.The best RFI shielding is normally an electrically connected andgrounded box that completely encloses the RF coil, such that externalnoise cannot reach the RF coil directly. For most real-worldapplications of this technology, this arrangement is not alwayspossible. An RFI trap or cut-off device is needed to permit access toone or both ends of the coil for movement of the sample item in and outof the coil. In an application of this invention, a portable hand-fedmail or package scanning device, only one end is open and this end, door85 (FIG. 2), is closed after the package is inserted and before the testis commenced. This closed configuration completes the RFI trap.

For a conveyor system to scan airline baggage, both ends of the RFshield are, of necessity, open, as shown in FIG. 4. To provide thenecessary RFI shielding, a tunnel, commonly known as a “wave guide belowcut-off” of about the same maximum cross-sectional dimension as thecoil, is required. Ends 66 and 71 provide the wave guide below cut-offfor this configuration of the invention. While the overall dimensionsare greater, the coil, shield and opening relationships remainsubstantially consisted. In this case the “X” dimension between coil 61and shield 62 is X/2, the same as between edge 63 of the coil and end 64of the main part of baggage-size scanner structure 67. As an example,the X dimension may be 45.7 cm (18 inches) and the width, dimension “Y,”could be around 71 cm (28 inches). Opening 65 is the same size all theway through wave guide or tunnel end extension 66, the main tunnel (notshown) through test apparatus box 67, and out through wave guide ortunnel end extension 71.

Some additional exemplary dimensions are given here for purposes ofcompleteness. The front-to-back length of the cavity, in scanner box 67,is about 89 cm (36 inches) and the cavity volume would be about 305liters (10.5 ft³).

While X/2 is the preferred sparing discussed above, it need not haveexactly that relationship to the short dimension of the coil cavity. Theshield spacings may range between X/3 and X, with X/2 being preferred atthe present time.

In addition to the coil and shield, some typical materials for facing 53and for inner rectangular frame 51 are wood and plastic. They should berelatively light, rigid, and be an electrical insulator. In the larger,double open-ended version of FIG. 4, external surfaces 72 and 73 oftunnel ends 66 and 71 would likely be copper or aluminum, while theinside and facing would be plastic or wood.

A practical smaller size, or portable, mail scanner 81 is shown in FIG.2. The electronics and control functional elements can be contained inbox 82. The scanning device itself is box 83 mounted on top of box 82 bystand-offs 84. The front of scanner 83 is normally closed by front dooror lid 85 which is hinged to the top box. A package 86 is shown inopening 52, within the RF coil, ready for test.

To reiterate, in the portable mail scanner of FIGS. 2 and 3, only oneend of the coil is required to be open for access to the coilcompartment or cavity. The shield entirely surrounds the coil, exceptfor an opening of the same cross-sectional area as the coil. Thisopening forms a slot through which packages can be passed. The openingin the shield is positioned in such a way (approximately 5.1-7.6 cm (2to 3 inches) from the end of the coil) that the magnetic flux from thecoil is “forced” to be contained within the shield itself. Thus, littleflux escapes from the shielded opening and little flux can enter thecavity. In order to further minimize EMI/RFI noise entry, a secondaryshield in the form of a grounded, aluminum enclosure, with EMI/RFIgaskets and a lid which overlaps the coaming may be employed. This is analuminum casing closely surrounding shielding 37 and does not add to theoverall dimensions, except to make overhang 68 if desired (see FIG. 3)This overhang is both aesthetically pleasing and provides improved RFIshielding for external RF. Lid 85 covers opening 52 with package 86 inthe cavity. Suitable rubber gaskets impregnated with conductive materialcompletes the EMI/RFI shielding.

Once the auto-tuning procedure has been completed, the scanningprocedure begins. The scanning procedure is standard for detecting NQRsignals in real-world detection applications. In one application of thisinvention, the procedure consists of a combination of RF pulses,commonly know as PAPS (phase-alternated pulse sequence) and NPAPS(non-phase-alternated pulse sequence) versions of the SSFP (steady statefree precession) pulse sequence. These sequences are described in U.S.Pat. No. 5,365,171, which is incorporated herein by reference to theextent necessary for full explanation. However, there are othersequences of RF pulses which are commonly used in NQR procedures whichare also applicable for use in this invention. These are known andreadily useable by those of ordinary skill in this technical field.

When test button 87 is pushed, the coil is tuned and scanning of thepackage is accomplished and at least one of the lights is illuminated.White light 111 flashes while tuning and testing are being completed.Illumination of green light 112 indicates that no contraband beingtested for is present. Illumination of red light 113 indicates that thetarget substance has been found in such a quantity as to be significant.If yellow light 114 is illuminated, it means there may be somethingpresent which should be looked at or further tested. It could mean thereis a significant amount of metal present. Both yellow and green lightsilluminated means thee was no clear NQR signal and there was metal orother conductive material present. Both red and yellow lightsilluminated indicates that the target substance may be present, but itis at least partially obscured by metal. Those are indeterminateresults. Not shown is an ON/OFF button on a non-visible side of unit 81.

One challenge which must be overcome in proceeding from the laboratoryto a practical NQR detection system for scanning airline baggage is thatof acoustic ringing. A standing wave can be set up in a conductor placedin a pulsed RF field. This acoustical wave is picked up by the RF coil.The signal produced is often close to the same magnitude andsufficiently close in characteristics to an NQR signal to possibly causea false alarm. The acoustical signal is often coherent with the excitingRF pulse, and hence can potentially be mistaken for an NQR signal, whichis also coherent with the exciting RF pulse. Moreover, common methodsfor reducing spurious ringing effects in laboratory NQR systems, such assignal averaging and/or reversing the RF phase, will often notsufficiently reduce the problem. Certain types of commonly-occurringmaterials, such as spring steel, are particularly prone to acousticringing.

In the preferred embodiment of this invention, a simple but effectivemethod of reducing the effects of acoustic ringing in NQR detectionapplications is employed. The primary differing characteristic of an NQRsignal compared with an acoustic ringing signal is that NQR signalsoccur only at pre-defined frequencies. Acoustic ringing signals, on theother hand, can be generated by any frequency of an RF excitation pulse.Thus, by operating the NQR scanning system at a frequency outside therange of the NQR sample frequency, using a standard or modified RF pulsesequence, no signal will be generated by or be detected from any targetmaterial. Under these conditions, if a signal is seen, it is fromacoustic ringing. Implementation of this method is straightforward. The“ring detect” sequence can be implemented before or after the mainsample detect sequence and is part of the programming and RF signalgeneration. This frequency excursion can easily be provided by theauto-tune aspect of this invention.

As an alternative for detection of acoustic ringing, the standard targetsubstance detection scanning cycle can be employed. It is a principal ofacoustic ringing that the ringing signal decays with time. Within alimited time period, between respective RF pulses, the NQR signalincreases with time. This feature can be used to determine the nature ofthe signal response. This procedure can be used in some instances, andis limited at the highest sensitivity levels, where the noise level ofthe system is comparable to the signal level.

In the package or mail scanner configuration of this invention, whenemploying analog detectors, signal capture and data processing subsystem41 comprises two analog to digital (A/D) converters 42 and 43 anddigital signal processor 44. The received signals from phase sensitivedetectors 27 and 28 are fed to A/D converters 42 and 43 respectively.All signals produced by the sample scan and ring detect sequences arefed into the A/D converters and are processed by the digital signalprocessor. Though the sample scan sequence, signals are either added orsubtracted, according to the algorithm outlined in U.S. Pat. No.5,365,171. The additions/subtraction algorithm reduces the effects of RFcoil ring-down and magnetoacoustic ringing.

In a practical configuration of this portion of the invention, signalcapture and most of the signal processing is carried out on a plug-in PCA/D converter card. The card has two channels, 14-bit resolution, and a2 MHz sampling rate. Subsystem 41 also performs on-board digital signalprocessing functions, such as addition or subtraction of consecutivedata sets as required. Once processing the output signal is completed,it is digitally filtered and compared to a predefined threshold level.Alternatively, once the signal is apodized and Fourier-transformed, itoccurs as a quadrature “spike” at or close to 0 Hz in the frequencyspectrum, and is then filtered and compared to the “known” signal of thematerial to be detected.

In the frequency domain, the signal capture and data processingsubsystem compares other signal factors to the expected signal factors.For example, it may compare the signal shape (Lorentzian or Gaussian) tothe line-width at half height. A combination of the above signal factorsmay be used to determine the presence or absence of the targetsubstance. The output of the digital signal processor is then sent todisplay device 46.

The NQR detected signal is compared with a predetermined threshold levelstored in memory in digital signal processor 44. If the detected signalis equal to or greater than the predetermined threshold, red light 113flashes on the operator's panel on display device 46, indicating thepresence of the target substance. If the signal is less than thepredetermined threshold, green light 112 flashes, indicating the absenceof the target substance. If the auto-tune algorithm detects that anexcessive amount of re-tuning of the coil is necessary, compared to anaverage investigation or predefined thresholds, or an acoustic ringingsignal is detected, the condition is flagged and yellow warning light114 illuminates. The yellow warning light indicates that: (1) there isan abnormally high amount of metal in the coil, (2) a high quantity ofhigh dielectric material is detected, or (3) a spurious acoustic signalhas been detected. Further alternative testing or visual inspection canbe used to resolve inconclusive results of the NQR test.

In addition to the illumination indications mentioned above, the displaydevice can optionally provide graphical display 95 of the signal showingboth the in-phase and quadrature signals, as well as other signal andsystem characteristics. Also optionally, printed output 96, includingthe time, date, signal amplitude and frequency, as well as coil tuningparameters, and other information such as acoustic signal responses fromspeaker 97, can be provided.

The factors which have degraded the effectiveness of previous NQR signaldetectors are reduced or eliminated by this system. If conductive orhigh dielectric materials are present in the sample, the auto-tunesub-system will be employed in an attempt to neutralize the effect ofthe foreign material. Then visual inspection can be accomplished ifthere is reason to do so. The auto-tune capability can quickly accountfor changes in temperature which affects tuning capacitance, as well asmovement or distortion of the coil which might occur when samples areput into the cavity.

Examples of two embodiments of the invention have been described above.It is likely that modifications and improvements will occur to thoseskilled in this technical field which are within the scope of theappended claims.

We claim:
 1. A system for detecting a target substance within a class ofexplosives and narcotics containing quadrupolar nuclei in a specimenemploying the phenomenon of nuclear quadrupole resonance (NQR), saidsystem comprising: a sequence controller configured to provide timingand programming pulses to said system; a radio frequency (RF) subsystemcomprising a variable frequency RF source to provide pulsed RFexcitation at a frequency generally corresponding to predeterminedcharacteristic nuclear quadrupolar resonant frequency of the specimen,wherein nuclear quadrupolar resonant frequency is the frequency ofnuclear precession due to quadrupolar interaction with molecularelectric field gradients; a detection subsystem comprising: adistributed RF coil sheet shaped and configured to define a cavity ofpredetermined volume therein and to receive the specimen within thecavity defined by said RF coil, said cavity having a first end and asecond end, the RF signal from said RF source being transmitted withinsaid cavity and being applied to the specimen within said RF coilcavity, said RF coil also functioning as the pickup coil for the NQRsignals from the specimen and providing an NQR output signal; apparatusfor adaptively tuning said RF coil to about the desired characteristicnuclear quadrupolar resonant frequency for the specimen under test; andan electrically conductive RF shield surrounding and spaced from said RFcoil, said RF shield being shaped and configured to provideelectromagnetic interference and radio frequency interference (EMI/RFI)shielding from external noise and to prevent RF and magnetic flux fromescaping from said RF coil cavity and RF shield combination, said RFshield being an electrically integral part of said RF coil and beingshaped and configured to improve the Q and the efficiency of said RFcoil and contributing to the effectiveness of the flux field applied tothe specimen within said RF coil, said RF coil and RF shield togetherforming a scanner; a signal capture and data processing subsystemcomprising: a receiver for the NQR output signal from said RF coil; amemory which stores characteristics of NQR signals from at least onetarget substance in the class of explosives and narcotic compounds; aprocessor for the NQR output signal from said RF coil; and a comparatorfor comparing characteristics of the processed NQR output signal withthe characteristics in memory and emitting a final output signal; and anindicating device receiving the final output signal from said signalcapture and data processing subsystem and, in response thereto, saiddisplay device indicating the presence of the target substance.
 2. Thesystem recited in claim 1, wherein said RF coil cavity is configured toleave at least one end accessible, through which the specimen can beinserted.
 3. The system recited in claim 2, wherein both ends of saidcavity are open to enable specimens to enter at one end and leave at theother end in a continuous manner.
 4. The system recited in claim 1,wherein said cavity is rectangular in shape, with its shorter sideshaving a first dimension, the spacing between said RF coil and said RFshield being in the range of about one-third of said first dimension tothe full length of said first dimension.
 5. The system recited in claim4, wherein the spacing between said RF coil and said RF shield is aboutone-half of said first dimension.
 6. The system recited in claim 4,wherein said RF shield extends past at least the open end of said RFcoil by a distance that is substantially equal to one-half of said firstdimension.
 7. The system recited in claim 1, and further comprising atleast one cavity extension element coupled to said scanner andcomprising a wave guide below cut-off, said extension element having anopening therethrough which is about the same size as said cavity.
 8. Thesystem recited in claim 1, and further comprising a cavity extensionelement coupled to each end of said scanner and having an openingtherethrough which is about the same size as said cavity, said extensionelements comprising wave guides below cut-off.
 9. The system recited inclaim 2, wherein said cavity is rectangular in shape, with its shortersides having a first dimension, the spacing between said RF coil andsaid RF shield being in the range of about one-third of said firstdimension to the full length of said first dimension.
 10. The systemrecited in claim 9, wherein said RF shield extends past at least theopen end of said RF coil by a distance that is substantially equal toone-half of said first dimension.
 11. The system recited in claim 1,wherein said adaptive tuning apparatus comprises means for automaticallytuning said RF coil after the specimen is inserted therein for maximumpower transfer efficiency.
 12. The system recited in claim 11, whereinsaid automatic tuning means comprises: a series of fixed valuecapacitors switched by controllable switch means; and control means forcontrolling the switching sequence of said capacitors to establishmaximum power transfer efficiency of said RF coil.
 13. The systemrecited in claim 12, wherein each successive capacitor has a capacitivevalue which is a power of two greater than the previous one.
 14. Thesystem recited in claim 12, wherein said controllable switch meanscomprise a series of vacuum relays, one for each said capacitor, saidvacuum relays being individually controlled by said control means. 15.The system recited in claim 1, and further comprising means to identifyand differentiate acoustic ringing signals from NQR signals.
 16. Thesystem recited in claim 15, wherein said identifying and differentiatingmeans comprises said sequence controller and said RF source, incombination with said detection subsystem to apply excitation RF pulsesto said RF coil which are separate and distinct from the RF excitationpulses at the quadrupolar resonant frequency.
 17. The system recited inclaim 1, wherein said RF coil is formed on an insulator frame with alinear gap between confronting edges of said RF coil sheet, saiddetection subsystem further comprising: a multiplicity of static tuningcapacitors connected between said RF coil confronting edges, said statictuning capacitors being configured and spaced to provide an RF fluxfield of predetermined characteristics within said RF coil cavity whenexcited by RF pulses.
 18. The system recited in claim 17, wherein saidstatic tuning capacitors are so spaced that capacitance values areconcentrated and spread farther apart based upon the NOR resonantfrequency of the target substance.
 19. A method for detecting a targetsubstance within a class of explosives and narcotics containingquadrupolar nuclei in a specimen, said method employing the phenomenonof nuclear quadrupole resonance (NQR) in a detection system andcomprising the steps of: forming a scanner comprised of a distributed RFcoil sheet shaped and configured to define a cavity of predeterminedvolume therein and to receive the specimen within the cavity defined bythe RF coil, the RF coil being surrounded by an electrically conductiveRF shield which is spaced from the RF coil, the shield being shaped andconfigured to provide electromagnetic interference and radio frequencyinterference (EMI/RFI) shielding from external noise and to prevent RFand magnetic flux from escaping from the scanner, the RF coil and RFshield being an integral combination portion of the scanner and beingshaped and configured to improve the Q and the efficiency of the RFcoil, the RF coil and RF shield contributing to the effectiveness of theflux field applied to the specimen when it is inserted within the RFcoil cavity; entering known characteristics of NQR signals of targetsubstances in memory in a signal processor in the detection system;providing programmed timing pulses to the detection system; insertingthe specimen within the cavity formed in the RF coil; then automaticallyadaptively tuning the RF coil to maximum power transfer efficiency forRF signals transmitted within the RF coil cavity; providing excitationRF pulses of a predetermined frequency to the RF coil; transmitting theRF pulses into the cavity formed by the RF coil and creating a fluxfield with the RF coil to which the specimen is subjected; detecting bythe RF coil the NQR signals emitted by target substances within thespecimen; processing the NQR signals and comparing them to known signalcharacteristics to determine whether the detected NQR signals indicatethe presence of a target substance; and indicating whether the targetsubstance is present in the specimen.
 20. The method of claim 19, andcomprising the further step of identifying acoustic ringing signals anddifferentiating said ringing signals from NQR signals.
 21. The methodrecited in claim 20, wherein said identifying and differentiating stepcomprises the steps of applying an RF signal to the RF coil which isseparate and distinct from the NQR excitation signal, and determining ifa ringing signal is present.
 22. The method recited in claim 19, whereinsaid indicating step further provides indications whether conditions ofthe received signal indicate that further examination is necessary. 23.The method recited in claim 19, wherein said RF coil is formed around aninsulative frame with a linear gap between confronting edges of the RFcoil sheets, the confronting edges being coupled by means of statictuning distributed capacitance in the form of spaced fixed valuecapacitors, the spacing of the static tuning capacitors being determinedby: exciting the RF coil to about the target resonant frequency; sensingthe RF flux field intensities at locations throughout the RF coil cavityto map the relative field intensities; linearly adjusting thecapacitance in the gap; re-mapping the field within the RF coil cavityand readjusting the gap capacitance to achieve a field within the RFcoil cavity of predetermined characteristics when the RF coil is excitedby RF pulses.
 24. The method recited in claim 23, wherein thecapacitance is adjusted by changing relative locations of the statictuning capacitors in the gap based upon the NOR resonant frequency ofthe target substance.